This invention relates to the field of devices intended to create a difference signal between two optical signals. The invention is particularly applicable to a device for reconstituting an optical signal for transmission of data, and particularly a rectangular type signal, for example an encoded signal without a return to zero. The invention is also applicable for the generation of a clock signal at twice the frequency of the clock frequency of a first signal.
An article by H. K. LEE et al entitled xe2x80x9cAll fibre optic clock recovery from non return to zero format dataxe2x80x9d published in the xe2x80x9cElectronics Lettersxe2x80x9d journal vol. 34 No. 5, March 1998 (document 1) describes a device for reconstituting a clock signal starting from an optical signal for transmission of data encoded in a code without a return to zero (NRZ code).
A part on the input side of the device described in this article is an optical fibre differentiator that generates a signal with a pseudo return to zero (PRZ), starting from the signal in the NRZ code. The PRZ signal thus built up from the NRZ signal is then used in a known manner to lock self-oscillating means. In the case described in the article, it is a laser cavity in locked mode comprising a non-linear optical loop mirror (NOLM).
The input side differentiator device comprises an asymmetric Mach Zehnder interferometric structure with two arms, one comprising a 300 ps delay xcfx84 in the form of a 6 cm additional fibre length. The NRZ signal is input into each arm of the asymmetric Mach Zehnder interferometric structure by means of a 3 dB coupler into which the NRZ signal is input. For the rest of the presentation, it is important to note that in the experimental device described in this article, the NRZ signal was generated in place by means of a tuneable laser diode in which the continuous output wave was modulated in a modulator into which an NRZ modulation signal output by a generator of this type of signal, was input. For a good understanding of the rest of the presentation, it is also important to note that the delay xcfx84, as described in the article in column 1 on page 479, represents the width of pulses forming the differential output signal at 3 dB. This delay xcfx84 must be equal to an odd number of half periods of the continuous carrier wave, if the destructive condition for signals present in each of the arms of the asymmetric Mach Zehnder interferometric structure is to be satisfied. To obtain this result, either the wavelength of this carrier must be varied as explained in the article at the top of column 2 on page 479 until the destruction condition is obtained, or the fibre length causing the delay between the signals propagating in each of the arms, must be varied. For obvious reasons of ease of construction, the authors chose a continuous wave generation diode, tuneable with sufficient resolution to obtain a wavelength adjustment capable of creating a phase shift satisfying the destruction condition.
The experimental device described in this article was used to obtain a PRZ signal starting from an NRZ signal at a rate of 1.5 Gigabits per second. This PRZ signal is then used to lock a clock signal reconstituting the clock signal from the NRZ signal.
Note that in the experimental device described in this article, the wavelength of the signal carrier wave is available in place and therefore that it is easy to act on it to adjust it and thus obtain the destructive condition assuming a phase shift of (2k+1)xcfx80 between the signals circulating in each of the arms of the interferometer.
It is difficult to create an industrial application of the device described in this article, since in practice it is required to reconstitute the clock signal at a regenerator starting from a carrier wave of an NRZ signal for which the wavelength is not known in advance. Furthermore, the stability of the carrier wave may not be sufficient to guarantee the destruction condition in the long term. This is why there is a need for a device capable of differentiating two signals, one of which is delayed with respect to the other, in other words a device in which the delay between the two signals can be controlled to maintain an operating difference that is equal to or close to (2k+1)xcfx80 at all times.
According to the invention, the problem of the phase adjustment between the first and a second signal lagging behind the first signal, to satisfy the destructive interference condition, is solved by a device in which there is a means for creating a continuous wave. This continuous wave is sent in a first channel comprising a medium with a refraction index n that is variable as a function of a characteristic of the signal, for example the frequency or the optical power passing through the medium. This same medium that has a refraction index n, is variable as a function of a characteristic of the signal, for example the frequency or the optical power passing through it receives the first signal such that the index n of the medium is modulated by the high and low levels of the characteristic of the first signal. This continuous wave is also sent in a second channel comprising a medium in which the refraction index n is variable under the same conditions. This same medium in the second channel receives the second signal such that the index n of the medium is modulated by the high and low levels of the second signal. For example, by modulating the power level of the first and/or the second signal, the index n of the first and/or second medium is modified, and therefore the time taken by the continuous optical wave passing through these media, to pass through this medium, is modified. Therefore, the delay of one of the channels with respect to the other can be adjusted to obtain a destructive beat between the first and the second signal. When the continuous wave that followed the first channel and the continuous wave that follow the second channel are made to interfere, the phase shift between these two waves is equal to xcfx80, and the difference between the wave modulated by the first signal and a wave modulated by the second signal is determined. Therefore, this gives a signal representative of the difference between the first and second signals.
In summary, the invention relates to an optical device for the differentiation of two optical signals, a first signal and a second signal, the second signal being the same as the first signal but lagging behind it by a delay xcfx84 comprising:
two channels, a first and a second channel, the first channel comprising a delay means for delaying the first signal input into this channel by xcfx84, the delayed signal forming the second signal,
means of generating a continuous wave,
device characterised in that it comprises:
a first medium and a second medium, with optical propagation indexes that vary with a characteristic of the optical signal passing through the said medium, placed on the first and second channels respectively,
means of inputting firstly the first or the second signal and secondly the continuous optical wave into the first medium, and of inputting firstly the first signal and secondly the continuous optical wave into the second medium,
means of making the first and second signals output from the first and second media respectively interfere with each other, a signal present at the output from these interference means making up the difference signal between the first and the second signal.
In the device according to the invention, the phase shift between the first and the second signal is independent of the wavelength of the signal carrier wave.
Delay means delaying the first signal by xcfx84 may be placed indifferently on the input side or output side of the variable index optical medium. Considering the selected vocabulary convention, if the delay xcfx84 is on the input side of the first medium the second signal is input into the first medium, whereas if the delay xcfx84 is on the output side of the first medium the first signal is input into the first medium.
Any means of multiplexing a signal input onto a channel and distributing it between two channels may be used to input the first signal onto the first and second channels, for example a 3 dB coupler or a multimode interferometric structure; the same is true for the interferometric output structure located on the output side of the first and second media.
In the preferred embodiment, the index of the first and second media varies as a function of the optical power that passes through them and are optical semiconductor amplifiers. A phase delay adjustment is obtained by adjusting the polarization current of the amplifier, modifying the amplifier gain and therefore the power level passing through the optical medium. The variation of power passing through the optical medium causes a variation of the index of this medium. Thus, it can be seen that the adjustment of the gain creates a variation of the propagation time. In this embodiment, the phase delay is preferably controlled in a closed loop in order to minimise the average level of the difference signal.